This application claims the Paris convention priority of Japanese Patent Application No. 11-218641/1999 filed on Aug. 2, 1999, which is incorporated herein by reference.
The present invention relates to a sealed alkaline storage battery.
Nickel positive electrodes of sealed alkaline storage batteries are divided into two types: a sintered electrode and a nonsintered electrode. A sintered nickel positive electrode, which uses a sintered substance of a metal as a conductive substrate (current collector), has a problem derived from low porosity of the sintered substance that an active material can be packed in merely a small amount, namely, the energy density is low. Therefore, attention has been recently attracted to a nonsintered nickel positive electrode that uses, as the conductive substrate, a nonsintered substance with high porosity such as nickel foam so that the active material can be packed in a large amount.
The nonsintered nickel positive electrode, however, has a problem of poor active material utilization. One of the reasons for the poor active material utilization of the nonsintered nickel positive electrode is as follows: Part of nickel hydroxide is changed into xcex3-NiOOH with low apparent density during charge so as to expand the electrode. As a result, a separator is compressed by the electrode, and hence, lack of an electrolyte (dry out) is caused in the separator, which increases the internal resistance of the battery.
Japanese Laid-Open Patent Publication No. 5-21064/1993 describes that xcex3-NiOOH can be suppressed from being produced during charge by using nickel hydroxide including 1 through 7 wt % of manganese as a solid-solution element and using an electrolyte (with specific gravity of 1.23 through 1.40) in amount of 1.0 through 2.0 cm3 per 1 Ah of the battery capacity. The publication reports that an alkaline storage battery can thus attain a long charge-discharge cycle life.
It has been found, however, as a result of examination made by the present inventors, that the alkaline storage battery described in this publication is poor in the charge acceptance because of its small content of manganese in nickel hydroxide and that the dry out occurs in a small number of cycles because of the small amount of electrolyte. The content of manganese as a solid-solution element of 7 wt % corresponds to approximately 11 wt % when converted into a ratio of the amount of manganese to the total amount of nickel and manganese. Also, the amount of electrolyte of 2.0 cm3 per 1 Ah of the battery capacity corresponds to 0.53 g at most (namely, in an ideal battery having a battery capacity equal to the theoretical capacity) when converted into the amount of water per gram of nickel hydroxide.
Accordingly, an object of the invention is providing a sealed alkaline storage battery that can exhibit high active material utilization for a large number of charge-discharge cycles owing to its good charge acceptance and minimal probability of occurrence of the dry out.
The sealed alkaline storage battery of this invention (present battery) comprises a nonsintered nickel positive electrode using nickel hydroxide as an active material, a negative electrode and an alkaline electrolyte, and the nickel hydroxide is manganese-containing xcex1-nickel hydroxide including, as a solid-solution element, 15 through 50 wt % of manganese on the basis of a total amount of nickel and manganese, and the alkaline electrolyte includes 0.55 through 0.80 g of water per gram of the manganese-containing xcex1-nickel hydroxide.
As a result, the invention provides an alkaline storage battery that can exhibit high positive electrode active material utilization for a large number of charge-discharge cycles.
In the present battery, manganese-containing xcex1-nickel hydroxide including, as a solid-solution element, 15 through 50 wt % of manganese on the basis of the total amount of nickel and manganese is used as a positive electrode active material. When the content of manganese is out of this range, the oxygen overvoltage of the manganese-containing xcex1-nickel hydroxide is so low that the charge acceptance and the active material utilization of the positive electrode cannot be sufficiently improved. The manganese-containing xcex1-nickel hydroxide is oxidized through charge so as to be changed into manganese-containing xcex3-NiOOH. The manganese-containing xcex3-NiOOH including 15 through 50 wt % of manganese has much higher oxygen overvoltage (corresponding to a difference between the oxygen evolution potential and the oxidation potential) than xcex2-NiOOH produced by charging xcex2-nickel hydroxide. Accordingly, the present battery attains high charge acceptance of the positive electrode and high positive electrode active material utilization at the initial stage of the charge-discharge cycles.
The present battery uses the alkaline electrolyte including 0.55 through 0.80 g of water per gram of the manganese-containing xcex1-nickel hydroxide. A conventional alkaline storage battery using a nonsintered nickel positive electrode generally uses an alkaline electrolyte including 0.20 through 0.50 g of water per gram of nickel hydroxide. The xcex3-NiOOH produced through charge of the xcex1-nickel hydroxide has, however, low apparent density. Therefore, the nonsintered nickel positive electrode of the conventional battery expands during charge so as to compress the separator, and hence, the dry out can easily occur in the separator. The dry out can increase the internal resistance of the battery, which degrades the charge acceptance and the active material utilization. Accordingly, in order to suppress the dry out caused because of the production of the xcex3-NiOOH, the present battery uses a larger amount of alkaline electrolyte than the conventional battery. The water content of the alkaline electrolyte should be 0.55 through 0.80 g per gram of the manganese-containing xcex1-nickel hydroxide for the following reason: When the water content is smaller than 0.55 g, it is difficult to suppress the dry out. When the water content exceeds 0.80 g, the volume of a space in the battery can is so small that the internal pressure of the battery can be easily increased during charge, resulting in easily causing leakage of the alkaline electrolyte.
A specific example of the nonsintered nickel positive electrode is a pasted nickel electrode obtained by coating a conductive substrate with a paste including an active material and drying the resultant. Specific examples of the conductive substrate are foam nickel, a felted nickel fiber porous material, punching metal, and a foamed metal of iron or the like coated with nickel plating or the like. Examples of the nonsintered nickel positive electrode other than the pasted nickel electrode are a tubular nickel electrode obtained by packing an active material in a tubular metallic conductive substance, a pocket nickel electrode obtained by packing an active material in a pocket-like metallic conductive substance, and a nickel electrode for a button-type cell obtained by compressedly molding an active material and a reticular metallic conductive substance.
Examples of the negative electrode of the present battery are a hydrogen-absorbing alloy electrode, a cadmium electrode and a zinc electrode.
Embodiments